This invention relates to pressurized water nuclear reactors. It relates particularly to the construction of the core barrel and upper support plate assembly used in a pressurized water nuclear reactor.
A pressurized water nuclear reactor power station contains a closed loop of pressurized water which removes heat energy from a nuclear core and transfers the heat energy to a secondary water system used to generate steam. The steam, in turn, drives a turbine generator set to produce electric power. The nuclear reactor system is comprised of a pressure vessel containing the nuclear fuel which generates the heat energy, a steam generator in which the heat energy from the nuclear reactor is used to generate steam, a circulating pump which circulates the coolant and a pressurizer that maintains and controls the pressure within the system.
The nuclear reactor assembly is generally comprised of a steel pressure vessel having a body and a removable top head, a thermal shield within the pressure vessel body, an upper support plate, several core support plates, a core barrel, and control rods and the nuclear fuel making up the core.
Certain components of the nuclear reactor assembly, after years of service, may develop primary water stress corrosion cracking. If such cracking occurs in pressure boundary components, the potential to generate leak paths for the pressurized water coolant exists. Such leak paths could lead to a leakage of coolant water outside the pressure vessel and into the surrounding containment building, thereby creating a potential hazard.
Alloy 600 is a material used in many reactor components. The reduction in temperature of the structural material is recognized as one method to reduce the susceptibility of a material to primary water stress corrosion cracking. The reactor vessel head is fitted with penetration tubes fabricated from ally 600. The penetration tubes allow access through the reactor vessel head for instrumentation and control mechanisms.
With new construction additional cooling manifolds and coolant passages can be built into various components to remove additional heat and reduce the temperature of the penetration tubes during operation. Such modifications would be quite difficult and expensive to use with nuclear reactors already in service, due to the fact that the interior of the reactor pressure vessel body, the removable top head and other internal components are radioactive.